For an 11-year-old weighing 20 kg with mild fever, cold, and cough, this is often a viral upper-respiratory infection, but I cannot diagnose online.
Use a pediatric formulation and check the concentration on the bottle before measuring. Avoid combining multiple cold/flu products because they may contain the same fever medicine.
Get urgent medical care now if there is fast or difficult breathing, chest pulling in while breathing, blue/gray lips, severe sleepiness/confusion, inability to drink, signs of dehydration such as very little urine or no tears, severe headache/stiff neck, rash with fever, seizures, or fever/cough that improves then becomes worse. CDC also advises review if fever lasts more than 4 days or symptoms last over 10 days without improving.
CDC warning signs